The present invention relates generally to the field of engine fuel mixture control apparatus and more particularly to the field of controlling the amount of air added to the engine fuel mixture by an air bypass valve (dashpot) which adds additional air to the fuel mixture in addition to the amount of air in the fuel mixture determined in accordance with engine throttle position.
It is known that during engine deceleration, defined herein as occurring in response to the abrupt effective closure of the engine throttle and corresponding to the release of pressure from the gas pedal for standard automotive gasoline engines, it is typically desirable to add additional air to the engine fuel mixture to encourage the vaporization of excess fuel such as gasoline during the deceleration transient. After the deceleration transient, the engine speed will arrive at a steady state "idle" speed corresponding to the existing engine load condition with the engine throttle effectively closed and no additional air being supplied by the air bypass valve. This corresponds to a minimum effective actuation of the dashpot. The additional air supplied during deceleration is required to minimize hydrocarbon emissions produced by the engine in response to the engine attempting to operate on too rich of a fuel mixture.
Prior engine fuel mixture control systems noted that additional air should be provided during engine deceleration and that the amount of this additional air should be decreased to zero after the deceleration transient when the engine was operating at its final idle speed. These prior systems implemented this function by providing for full dashpot actuation during deceleration for engine speeds above some maximum predetermined engine speed threshold level, and then, as the engine speed decreased below this level, gradually decreasing the degree of dashpot actuation as a fixed function of elapsed time or as a fixed function of an elapsed number of engine revolutions. In these prior systems the rate of decrease of dashpot actuation is primarily determined by the elapsed time which occurs once the engine speed has decreased below the maximum predetermined engine speed level. It should be noted that the number of engine revolutions is actually a product function of engine speed and elapsed time, and is primarily a function of time during such decelerations.
While such engine fuel mixture control systems as those described above are feasible, the present invention has recognized that controlling the degree of dashpot actuation primarily in accordance with elapsed time during deceleration does not provide the desired amount of dashpot actuation decrease under various different engine deceleration conditions.